in rustc_index: revert "fix compilation error by changing the rustc_serialize dependency to be required instead of optional", and introduce conditional compilation instead.

Author: Tage Johansson

compiler/rustc_index/Cargo.toml

@@ -8,14 +8,15 @@ edition = "2024"
 itertools = "0.12"
 rustc_index_macros = { path = "../rustc_index_macros" }
 rustc_macros = { path = "../rustc_macros", optional = true }
-rustc_serialize = { path = "../rustc_serialize" }
+rustc_serialize = { path = "../rustc_serialize", optional = true }
 smallvec = "1.8.1"
 # tidy-alphabetical-end
 
 [features]
 # tidy-alphabetical-start
 default = ["nightly"]
 nightly = [
+    "dep:rustc_serialize",
     "dep:rustc_macros",
     "rustc_index_macros/nightly",
 ]

compiler/rustc_index/src/bit_set/dense_bit_set.rs

@@ -8,6 +8,7 @@ use std::ptr::NonNull;
 use std::{fmt, iter, slice};
 
 use itertools::Either;
+#[cfg(feature = "nightly")]
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 
 use super::{
@@ -781,6 +782,7 @@ impl<T: Idx> BitRelations<ChunkedBitSet<T>> for DenseBitSet<T> {
     }
 }
 
+#[cfg(feature = "nightly")]
 impl<S: Encoder, T> Encodable<S> for DenseBitSet<T> {
     #[inline(never)] // FIXME: For profiling purposes
     fn encode(&self, s: &mut S) {
@@ -815,6 +817,7 @@ impl<S: Encoder, T> Encodable<S> for DenseBitSet<T> {
     }
 }
 
+#[cfg(feature = "nightly")]
 impl<D: Decoder, T> Decodable<D> for DenseBitSet<T> {
     #[inline(never)] // FIXME: For profiling purposes
     fn decode(d: &mut D) -> Self {